In general, there are two types of internal combustion engines, one being a reciprocating engine in which a piston performs linear strokes, and the other being a rotary engine in which the piston performs rotary motion.
The rotary engine includes a rotor operatively engaged with an engine output shaft, and a rotor housing accommodating therewithin the rotor. In a typical rotary engine, the rotor acting as a rotating piston is adapted to perform eccentric rotary motion within the rotor housing, in such a way that the engine output shaft is provided with an external toothed gear, which is engaged with the internal toothed gear of the rotor that is larger in diameter than the external gear of the engine output shaft, so that the eccentric rotation of the rotor is transmitted to the engine output shaft. The outline of the cross- section of the rotor sliding inner surface of the rotor housing is shaped to a substantially cocoon-shaped peritrochoid curve, and the cross-section of the rotor is shaped to a substantially equilateral triangle. The three vertex portions of the rotor slide on the peritrochoid curved inner surface of the rotor housing during the eccentric rotation of the rotor to form separate and independent compartments for suction, compression, explosion and exhaust, respectively within the engine compartment between the rotor and the inner surface of the rotor housing. Such prior art typical rotary engine avoids problems relating to the use of a suction valve and an exhaust valve related to the engine compartment, which are necessary for the reciprocating engine.
In such a typical rotary engine, during one rotor eccentric revolution, only one sequence of suction, compression, explosion and exhaust is carried out, and therefore there is such disadvantage that a high engine torque is not obtainable unless the operation of the engine becomes a high rotation speed. This disadvantage also exists in the case of the reciprocating engine. Further, a complicated mechanism is needed to cause the rotor to rotate eccentrically with respect to the engine output shaft, which results in increase in cost.
The idea of overexpansion, i.e. that an expansion volume is greater than a compression volume to increase engine efficiency, has been considered but is too impractical to implement in traditional reciprocating engines.